This invention relates generally to integrated circuits, and more particularly, to techniques for insulating multiple conductive layers in integrated circuit structures. In many integrated circuit designs, multiple patterned layers of conductive metal are used to interconnect the circuit components. The layers must be insulated from each other by a dielectric layer, but there is also a requirement that electrical contact may be made through the dielectric layer at selected locations. This is typically effected by means of openings in the dielectric, called via holes, the sidewalls of which are coated with metal to complete the electrical connection through the dielectric layer.
Silicon dioxide (SiO.sub.2) is probably the material most commonly used as a dielectric between metal layers. While satisfactory in most respects, silicon dioxide also has some shortcomings, one of which is that it does not always provide good coverage over steps in the planar topography of a wafer. Since a first of the metal layers will typically be patterned before the dielectric layer is formed, steps in the topography will be commonplace. Polyimide, which has been suggested as a substitute for silicon dioxide and other materials, has suitable thermal and electrical properties and is chemically inert. It has the additional advantage that it partially planarizes the wafer topography, and therefore improves the step coverage of overlying metal layers.
However, there are at least two major problems associated with the use of polyimide as an intermetal dielectric layer. One is that the opening of a via hole through the polyimide layer is rendered difficult by the production of a residue of polyimide, which accumulates at the bottom of the via hole, over the lower level metal layer. Unless the residue is completely removed, it can prevent good electrical connection between the metal layers. A second problem is that polyimide tends to absorb water, which may result in corrosion of the metal.
Prior to the present invention, the most reliable method of removing the polyimide residue was to sputter-etch the bottom of the via hole prior to the deposition of metal in the hole. However, this technique has been found not to work well for via holes of small dimensions, such as two micrometers by two micrometers or smaller.
Wet-etching of polyimide through a patterned photoresist layer is generally suitable for via sizes down to three micrometers in size. For smaller geometries, a dry etching process may be used. Unfortunately, dry-etching in an oxygen plasma, using a positive photoresist mask to define the via holes, is of only limited utility, since the oxygen plasma erodes the photoresist at about the same rate as it erodes the polyimide. A non-erodable or hard mask is required, as recognized by G. Samuelson in "Polyimide for Multilevel VLSI and Alpha Protection," from a course on "Polyimide Coatings for Microelectronics with Applications," Continuing Education in Engineering, Univ. of Cal., Berkeley, 1981.
Another consideration is that it is usually desirable to produce via holes with sloping sidewalls, since metallization of the hole will then be greatly facilitated. However, in order to ensure that all of the polyimide material has been removed from all of the via holes, it is necessary to include a prolonged "overetch" step. The risk in such a step is that some of the via holes may be enlarged too much in width.
It will be appreciated from the foregoing that there is still a need for improvement in techniques for using polyimide as a dielectric between metal layers. Ideally, the via holes in polyimide should be suitably shaped with sloped side walls, while retaining control over the feature sizes at the top and bottom of the via holes. In addition, the polyimide residue must be totally removed, to provide good electrical contact with the lower metal layer. It would also be desirable to preclude any possible moisture damage to the lower metal layer, which may be caused by direct contact with the polyimide. The present invention is directed to a technique with these ideal characteristics.